This invention relates generally to systems for determining the spatial position and angular orientation (i.e. pose) of a body, or object.
As is known in the art, systems are available for determining the spatial position and angular orientation of a body (or object). One such system includes passive retro-reflectors as point markers, or targets, affixed to the body and a second system includes active radiating emitters as the affixed point markers. Both techniques operate by projecting the image of a high contrasting marker onto spaced sensors and using mathematical processing to determine the three dimensional coordinates of each one of the point markers. These three dimensional coordinates (i.e., 3D) are then used as discrete points, or may be considered as a set if their geometric arrangement is known, resulting in the determination of the position and angular orientation of the body (i.e., six degrees of freedom: x,y and z positions and pitch, yaw and roll angular orientations) in space relative to a three dimensional coordinate system centered at a preselected point in space, typically at a point fixed relative to the sensors.
Determining the spatial position and either the vector angle or angular orientation of a body has several uses. For example, a pointing device can be made out of the body whereby the end tip of the pointing device is in a known position relative to the markers. Such a pointing device can be used as a digitizing pointer held by hand as in reverse engineering applications. An operator moves this pointing body to various known places on a manufactured component and the accuracy of the manufacturing processes is determined from analysis of the determined end tip position of the pointing device. This application requires a highly accurate system.
In another applications, such as in an image guided surgical procedure the instrument pose is being tracked with respect to the patient. Certain surgical instruments have affixed to them markers. This information can be used to allow the surgeon to see where the instrument is pointing on a MR or CT scan, and what is beyond the end tip of the surgical instrument. This application also requires a highly accurate system.
In one emitting marker (i.e., an active marker) system, multiple charge couple device (CCD) sensors are used to detect the energy emitted by the marker. A single point marker is energized per sensor cycle to emit infrared energy. During each sensor cycle, the emitted energy focused onto the sensor is collected (i.e. integrated) and shifted to the sensor processing circuitry. In order to determine the 3D position of the marker, the marker must be detected on at least three sensor axes (i.e. to cover a minimum of 3 orthogonal planes). There are many advantages to a system which uses emitting markers including high contrast images being produced on the sensors, control over activation of each of the markers affording positive and automatic marker discrimination, and the ability to use high speed linear sensors. High speed linear sensors are relatively expensive and only one marker can be tracked during a single sensor cycle.
In one retro-reflective marker (i.e., a passive marker) system, an energy source is energized to emit infrared energy in the general direction of the retro-reflective marker. Multiple CCD sensors are then used to detect the energy reflected by the marker. During each sensor cycle, the reflected energy focused onto the sensor is collected (i.e., integrated) and shifted to the sensor processing circuitry. In order to determine the 3D position of the marker, the marker must be detected on at least three sensor axes (i.e. to cover a minimum of 3 orthogonal planes). There are many advantages to a retro-reflective marker system including the use of wireless markers and the ability to use inexpensive low speed area array sensors. These systems, however, suffer from problems associated with positively identifying markers.
It is desirable to use a cost effective area array sensor which is capable of tracking multiple markers during a single sensor cycle. As is known in the art there are systems that make use of single area array sensor and inexpensive components. DeMenthon (Patent Number 5,227,985)teaches a system which uses a single sensor and matrix techniques to determine the pose of a body. This system is limited to noncoplanar markers and is based on projection methods to extract 6D information from 2D images. This method will not have sufficient accuracy for medical applications. As is well known in the art, the error in the depth measurement is prohibitively large for this type of system. Triangulation methods have a distinct advantage of projection methods for the depth accuracy. Triangulation methods, also called stereometric techniques, were rejected due to the costly hardware required to perform real-time calculations. Multiple marker triangulation methods with area array sensors have the additional problem of poor marker identification, which is typically solved with human intervention. Prior systems can operate poorly in the presence of real-life situations of stray IR sources and reflections that will appear to be unwanted and unexpected markers. Prior systems can also operate poorly in the presence of multiple bodies in close proximity to each other.